Thermal detectors operate by absorbing energy from electromagnetic radiation incident thereonto and by converting the heat thus generated into an electrical signal representative of the amount of absorbed radiation. Perhaps the most prominent type of thermal detectors currently available is uncooled microbolometer detectors, usually shortened as microbolometers. A microbolometer is typically based on a suspended platform or bridge structure having a low thermal mass and on which is disposed a material having a temperature-dependent electrical resistance. The platform is generally held above and thermally insulated from a substrate by a support structure, and is provided with a thermistor, which is the resistive element whose electrical resistance changes in response to temperature variations caused by the absorbed radiation. The thermistor may, for example, be composed of a material having a high temperature coefficient of resistance (TCR) such as vanadium oxide and amorphous silicon.
Microbolometers are capable of operating at room temperature. Because they do not require cryogenic cooling, may be integrated within compact and robust devices that are often less expensive and more reliable than those based on cooled detectors.
Arrays of uncooled microbolometer detectors may be fabricated on a substrate using common integrated circuit fabrication techniques. Such arrays are often referred to as focal plane arrays (FPAs). In most current applications, arrays of uncooled microbolometers are used to sense radiation in the infrared portion of the electromagnetic spectrum, usually in the mid-wave infrared, encompassing wavelengths of between about 3 and 5 μm (micrometers), or in the long-wave infrared, encompassing wavelengths of between about 8 and 14 μm.
Such arrays are often integrated in uncooled thermal cameras for sensing incoming infrared radiation from a target scene. Each microbolometer detector of the array absorbs some infrared radiation resulting in a corresponding change in the microbolometer detector temperature, which produces a corresponding change in electrical resistance. A two-dimensional pixelated thermal image representative of the infrared radiation incident from the scene can be generated by converting the changes in electrical resistance of each microbolometer detector of the array into an electrical signal that can be displayed on a screen or stored for later viewing or processing. By way of example, state-of-the-art arrays of infrared uncooled microbolometer detectors now include 1024 by 768 pixel arrays with a 17-μm pixel pitch.
In the last decade, there has been a growing interest toward extending uncooled microbolometer spectroscopy and sensing applications beyond the traditional infrared range, namely in the far-infrared and terahertz (or sub-millimeter) spectral regions. As known in the art, these regions of the electromagnetic spectrum have long been relatively unused for industrial and technological purposes due to the lack of efficient techniques for detection and generation of radiation in this frequency range.
In this context, extending the absorption spectrum of uncooled microbolometers beyond 30 μm is not straightforward, since the materials used to fabricate the detectors absorb predominantly in the infrared, and because the pitch of terahertz-sensitive pixels is typically larger than that of infrared-sensitive pixels to avoid diffraction effects. In addition, to maximize radiation absorption in the desired spectral band, conventional infrared microbolometer detectors generally include a reflector deposited on the underlying substrate to form a quarter-wavelength Fabry-Perot optical resonant cavity with the suspended platform. However, forming such a quarter-wavelength resonant cavity is generally not practical from the point of view of surface micromachining techniques used in the microfabrication of uncooled microbolometer detecting electromagnetic radiation at wavelengths longer than 10 μm.
Therefore, there remains a need in the art for an uncooled microbolometer detector capable of absorbing electromagnetic radiation in the terahertz and far-infrared regions, while retaining at least some of the advantages of infrared detector technology in terms of cost, reliability, ease of fabrication, and maturity of the field.